Over the past few years, I've run into several little "gotchas" when it comes to common moderation tasks such as migrating questions, reviewing posts, or evaluating suggested edits. These mistakes aren't as well documented as I'd like them to be.
What should I look out for when performing these moderation activities? (Feel free to edit the answer to add more information.)
Don't flag answers as "not an answer" unless they don't attempt to answer the question at all. If an answer post at least tries to answer the question, don't flag it, even if it is factually incorrect. Instead, you should downvote the answer.
The "very low quality" flag should be reserved for genuinely unsalvagsable posts. If an answer can be made useful through editing, don't flag it as very low quality. In addition, answers that are really comments or do not attempt to address the question should be flagged as "not an answer" instead.
The "spam" and "inappropriate" flags should be used only when a post contains blatant and unwelcome self-promotion, or content that an average person would find offensive. These flags help to train Stack Exchange's spam and abuse mitigation systems, and must be used appropriately. Don't use these to indicate the presence of poor quality, a lack of research, etc.
Don't migrate questions if they're on topic here. Even if the question is better on another site, if it is on topic here, don't migrate it. Instead, consider leaving a comment stating that the question may be better on another site, cautioning the user not to cross-post.
Don't migrate low-quality questions. If a question is on-topic for a particular site but would be closed there anyway, simply vote to close as off-topic without migrating.
Check for cross-posting. Users will sometimes cross-post questions on multiple sites in an attempt to get more answers. Especially with questions that are obvious candidates for migration, it's a good idea to check the post author's account on the destination site before voting to migrate.
Don't flag for migration to beta sites. Beta sites should mature on their own as much as possible, and migrating questions there would disrupt this process.
Editing and suggested edits
Check tag wiki suggested edits for plagiarism. Suggested edits for tag wikis are often copied from other sources; such edits should be rejected. A Google search for selected phrases in the edit, enclosed in quotes so that only results with the exact phrase are returned, can be used to check for plagiarism.
Don't edit spam or offensive posts. Again, spam and offensive flags are used to train Stack Exchange's spam filtering system. Editing the post will result in the edited content getting registered as spam rather than the actual spam content, distorting the filters. Instead, just flag spam or offensive posts as such.